Submitted to: Fungal Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 2, 2010
Publication Date: June 9, 2010
Citation: Widmer, T.L. 2010. Phytophthora kernoviae oospore maturity, germination, and infection. Fungal Biology. doi:10.1016/j.funbio.2010.06.001. 114:661-668. Interpretive Summary: Phytophthora kernoviae is a recently discovered pathogen on many different hosts including rhododendrons and beech trees. Although not currently known to exist in the United States, it is a pathogen of concern due to its destructive nature and threat to hosts in the U.S. It is believed that the propagule most likely to be involved in long distance dispersal is the oospore. Since very little is known about the basic biology of this pathogen, this study was conducted to understand the time needed to produce mature oospores and conditions under which they germinate and infect host material. Oospores formed at 10, 15, and 20 degrees Celsius by 6 days, but not at 25 or 28 degrees Celcius. Germination of oospores was optimum at 18 and 20 degrees Celcius under continuous light, but also had high germination in the dark. Rhododendron leaf disks developed disease symptoms when inoculated with oospores and kept in the dark but did not develop disease symptoms under continuous light. This study provides techniques that can be used by other researchers investigating P. kernoviae and provides results of initial studies that contribute to a better understanding of this pathogen.
Technical Abstract: Oospores of Phytophthora species are important survival structures that can initiate new infections. There is limited information on the basic biology of the recently described P. kernoviae, which produces oospores in single isolate cultures. In this study, three isolates of P. kernoviae originating from the United Kingdom (UK) and New Zealand were used to investigate oospore maturity, germination, and infection. All isolates produced oospores in V8 broth at 20 degrees C in the dark by 6 days. Oospores also formed at 10 and 15 degrees C, but did not form at 25 and 28 degrees C. Continuous light inhibited oospore production by UK isolates but had no negative affect on production by New Zealand isolates. Maximum oospore germination occurred when the cultures were between 6 and 10 weeks old. Germination of oospores was optimum at 18 and 20 degrees C, and did not occur at 5, 25, and 30 degrees C. When all isolates were combined for analysis, germination under continuous light (65.6 percent) was significantly higher than germination in the dark (52.0 percent). However, individual isolates showed variable results. Oospores of all isolates, except one isolate from the UK, infected and caused necrosis of Rhododendron leaf disks when inoculated in the dark at 20 degrees C. Inoculated leaf disks exposed to continuous light did not develop necrosis after 1 week, regardless of the isolate used. Infection of the leaf disks, as evidenced by surface sterilizing and plating on a selective medium (PARPH+V8), was lower in the leaves exposed to continuous light (40 percent) compared to those kept in the dark (100 percent).